196 UTILISATION OF FOOD BY THE ANIMAL [chap. 



the body, and the food supply must be sufficient to 

 repair this loss or the animal will be forced to consume 

 some of its previously stored-up fat or flesh. All 

 internal work, including that done during digestion, 

 reappears as heat, so that as long as the food is 

 above the grade of straw and contains more available 

 energy than is required for the digestive process, the 

 food is only called upon to repair the losses of heat and 

 maintain the body temperature ; the work required for 

 digestion, respiration, and other bodily processes being 

 performed by the way without interfering with the final 

 production of heat, because it is work done inside the 

 animal. In other words, the maintenance diet con- 

 tains just enough energy to drive the machine when it is 

 running idly with the animal at rest, and in the running 

 of the machine the food energy is transformed into heat 

 which keeps up the body temperature. Actually on 

 most diets, even maintenance rations, the energy re- 

 quired by the resting animal to maintain its temperature 

 is greater than that required for internal work ; some of 

 the food is burnt simply and solely as fuel for warming, 

 and the amount of food required will be determined 

 only by the heat lost by the animal. The magnitude 

 of this factor will vary with the size of the animal, or 

 rather with the surface it possesses, because heat is lost 

 only from the surface. Now the smaller the animal the 

 greater is its surface in proportion to its weight (a cubic 

 foot of water weighs 62-5 lb. and has a surface of 6 

 square feet ; a cube 2 feet on the side would weigh eight 

 times as much, but only have a surface of 24 square feet, 

 i.e. four times as much as before) ; hence small animals 

 will require for maintenance more food in proportion to 

 their weight than large ones. Of course, the greater the 

 difference in temperature between the animal and its 

 surrounding air the greater will be the loss of heat and 



